1. Field of the Invention
The invention relates to a distance measuring equipment (so-called laser radar) which is useful in, for example, a collision prevention system wherein a laser beam is forward emitted from a vehicle to measure a distance between the vehicle and a forward vehicle, and an alarm is produced when the present vehicle abnormally approaches the forward vehicle.
2. Description of the Related Art
A distance measuring equipment of this kind will be described with reference to FIG. 7.
In FIG. 7, reference numeral 1 designates a clock signal generator which supplies a timing clock signal to various circuits, and 2 designates a light emission trigger signal generator which converts the clock signal supplied from the clock signal generator 1 into a light emission signal functioning as a trigger signal, on the basis of a control signal supplied from a distance calculating circuit 15 which will be described later. Reference numeral 3 designates a driving circuit which is powered by a power source circuit 4, and which, in response to the supply of the light emission signal from the light emission trigger signal generator 2, drives at each supply a light emitting element 5 to generate a laser beam. The power source circuit 4 supplies the power not only to the driving circuit 3 but also to the other circuits.
Reference numeral 6 designates a light transmitting lens through which the laser beam from the light emitting element 5 is output so as to diffuse away from the element 5 at a predetermined angle, and 8 designates a light emitting unit which comprises the driving circuit 3, the power source circuit 4, the light emitting element 5, the light transmitting lens 6, and the like.
Reference numeral 9 designates a honeycomb filter which is disposed so as to cover the front surface of the incidence plane of a light receiving lens 10. Among light beams reflected by an object (not shown) which is in front of the equipment such as a forward vehicle (hereinafter, such an object is referred to as "forward object"), only light beams which are parallel to the optical axis of the light receiving lens 10 are allowed to pass through the lens, and light beams which are not parallel to the optical axis are disabled to pass the lens. Reference numeral 11 designates a light receiving element which receives light beams converged by the light receiving lens 10 and converts them into an electric signal, 12 designates an amplifier which amplifies the light reception signal from the light receiving element 11 and outputs the amplified signal, and 13 designates a light receiving unit which comprises the honeycomb filter 9, the light receiving lens 10, the light receiving element 11, the amplifier 12, and the like.
Reference numeral 14 designates an adder. In the case where a period is defined between the supply of a control signal from the distance calculating circuit 15 and the next supply of the control signal, each time when the control signal is supplied, the adder 14 adds the waveform output from the amplifier 12 in the current period to that in the previous period so as to convert the light reception signal into a signal having an improved S/N ratio, and then outputs the signal. Specifically, when the control signal is supplied N times from the distance calculating circuit 15 to the light emission trigger signal generator 2 and the adder 14, the light emitting element 5 emits a laser beam the corresponding number of times, i.e., N times, and waveforms respectively produced by the emitting operations and including the light reception signals output from the amplifier 12 are added to each other. When the adding operations are completed, the calculation result in the current period is binarized and then output.
The distance calculating circuit 15 supplies to the light emission trigger signal generator 2 the control signal for causing the light emitting element 5 to emit light. Further, the distance calculating circuit 15 instructs the adder 14 to conduct the adding operations, by means of the control signal.
When receiving the waveform showing the binarized addition result from the adder 14, the distance calculating circuit 15 calculates the distance between the equipment and the forward object on the basis of the resulting waveform. Specifically, in the pulse string showing the addition result which has been binarized at predetermined temporal intervals, a first pulse having a width which is greater than a predetermined level is detected, and the distance is calculated on the basis of the period between the time when the control signal is output and that of the first pulse.
Next, the operation of the equipment described above will be described.
When the power source for the whole of the equipment is turned on, the distance calculating circuit 15 repeatedly outputs the control signal (see FIG. 8(A)) at intervals of a predetermined period T. Each time when the control signal is output, the light emitting element 5 generates a laser beam, and the laser beam is forward emitted through the light transmitting lens 6.
The laser beam is reflected by the forward object to return to the equipment, and then detected by the light receiving element 11 through the light receiving lens 10. The output of the light receiving element 11 is supplied to the adder 14 via the amplifier 12. In the adder 14, the output from the amplifier 12 is divided for the predetermined period T, and, each time when a signal is newly obtained, the new signal is added to the signal of the previous period. In other words, the waveforms of N periods are added to each other (see FIG. 8(B)).
This allows the weakened reflection beams, i.e., the laser beams to be extracted as the light reception signal from the output of the amplifier 12, so that the laser beam is subjected to the signal processing.
Then the adder 14 binarizes the addition result to convert it into a logic signal (see FIG. 8(C)). The logic signal is checked to see whether or not the probability of the period of the high level with respect to that of the low level is equal to or greater than 50% in a unit period which is obtained by further dividing the period T. If the probability is equal to or greater than 50%, periods t1 and t2 of the maximum pulse widths in the high-level periods wherein the probability is equal to or greater than 50% are detected (waveforms P1 and P2 in FIG. 8(D)). The detected data (see FIG. 8(D)) are supplied to the distance calculating circuit 15.
As a result, the distance calculating circuit 15 calculates the distance between the forward object and the equipment on the basis of the temporal difference TO between the first timing P1, among the supplied data (see FIG. 8(D)), when the probability that the questioned pulse can be presumed a reflection signal is raised to 100%, and the timing when the distance calculating circuit 15 outputs the control signal. The calculated distance is supplied to an alarm judging circuit (not shown). When the distance enters a dangerous range, an alarm signal is produced. The second timing P2 when the probability is 100% is neglected.
In such a distance measuring equipment, however, a large current must be supplied when the light emitting element 5 such as a laser diode emits a light beam, and the light receiving element is required to have a high sensitivity. Each time when the light emitting element 5 emits a light beam, therefore, electric noise (see the portion Q1 of FIG. 8(B)) is superposed as a DC component on the output signal which is supplied from the amplifier 12 to the adder 14.
In the case where a signal (see the symbol Q2 of FIG. 8(B)) indicative of the reflection beam from the forward object and generated with being superposed on noise components close to white noise should originally be subjected to the signal processing so that the distance signal Td is detected as shown in FIG. 8(E), however, the adder 14 actually processes the noise portion Q1 as if it is a signal indicative of the reflection beam from the forward object. This results in that the distance is erroneously measured, thereby producing a problem in that the reliability of the measurement accuracy is lowered.
This problem may be solved by shielding the whole of the circuits or by disposing a noise filter in the power source circuit. When such a countermeasure is taken, however, the number of electronic parts is increased and the area for mounting these parts is enlarged and hence there arise further problems in that the production cost is increased, and that the equipment is too large to be used in a practical use.